Dear aspirant,
Mechatronics is a branch in engineering which focuses on the engineering of both electrical and mechanical systems, and also includes the combination of robotics , computer , telecommunications, systems, control and product engineering. 
While BCA Is a field of study which focuses on the computer applications , it is mainly the knowledge of the Information technology . You will learn about programming languages, databases, networking, data structure etc : . It is mainly focused on the computer field. 

So you can say that you have a variety of study in mechatronics rather than BCA .The placements in  MECHATRONICS is also way better than BCA . It will now depend upon you that what you want to study  , if you want to study only about computers then BCA is for you , if not then MECHATRONICS is for you simple as that.